Superalloys and heat resistant alloys are materials that exhibit superior mechanical and chemical attack resistance properties at elevated temperatures. Typically they include, as their main constituents, nickel, cobalt, and iron, either singly or in combinations thereof. In addition, other elements such as chromium, manganese, aluminum, titanium, silicon, molybdenum, etc., are added to improve the strength, corrosion resistance and oxidation resistance characteristics of the alloy. Inasmuch as these alloys are utilized in hot environments such as gas turbines, heat exchangers, furnace components, petrochemical installations, etc., their superior characteristics serve them well.
The properties of such alloys are strongly affected by their grain size. At relatively low temperatures, smaller grain sizes are acceptable. However, at higher temperatures (&gt;871.1.degree. C. or 1600.degree. F.) creep is usually observed to occur much more rapidly in fine grain materials than in coarse grain materials. Accordingly, coarse grained materials are usually preferred for stressed applications at elevated temperatures. It is believed that failure generally occurs at the grain boundaries oriented perpendicular to the direction of the applied stress.
One method used in improving the creep properties of an alloy is to attempt to elongate the grains. By elongating the grains, there are relatively fewer grain boundaries transverse to the stress axis. Moreover, longer elongated grain boundaries appear to improve the temperature characteristics of the alloy.
Oxide dispersion strengthened alloys made by mechanical alloying techniques exhibit superior high temperature rupture strength due to the presence of stable oxide particles in a coarse and highly elongated grain matrix.
However, depending on the circumstances, mechanically alloyed products may not always be required. Lower cost alloys, with intermediate properties (falling between wrought alloys and mechanical alloys) may be acceptable. Accordingly, it was believed that lower cost powder metallurgy alloys having intermediate properties could be produced by controlling the composition and oxide content (such as Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3) of atomized powders and applying suitable thermomechanical processing (TMP) steps to generate an alloy having a coarse, elongated grain structure.